Intervertebral prosthesis

ABSTRACT

An intervertebral prosthesis for insertion between adjacent vertebrae includes an upper plate, a lower plate and a core. The core is retained between the upper and lower plates by a retention feature in the form of central projections on the plates and a corresponding opening in the core. The retention feature is designed to allow the plates to slide over the upper and lower surfaces of the core in the anterior/posterior direction and in the lateral direction and to allow the plates to rotate with respect to each other and the core. The retention feature is also designed to prevent contact between the first and second plates during sliding movement of the plates over the core.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to medical devices and methods. More specifically, the invention relates to intervertebral prosthetic discs and methods for retaining a core in an intervertebral prosthetic disc.

Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.

Common causes of back pain are injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.

When a damaged intervertebral disc causes a patient pain and discomfort, surgery is often required. Typically, surgical procedures for treating intervertebral discs involve discectomy (partial or total removal of a disc), often followed by interbody fusion of the superior and inferior vertebrae adjacent to the disc or implantation of an intervertebral prosthetic disc. Fusion is most commonly achieved by implantation of a cage or spacer together with bone graft material to promote bone growth to fuse the adjacent vertebrae together. Oftentimes, pins, rods, screws, cages and/or the like are placed between the vertebrae to act as support structures to hold the vertebrae and bone graft material in place while the bones permanently fuse together.

While such fusion procedures have been very successful for many patients, it some cases the fusion spacers or cages can be difficult to implant, and the bone regrowth necessary to achieve complete fusion can take an excessive period of time. Additionally, fusion procedures limit patient mobility. Several types of intervertebral disc prostheses are currently available. For example, one type of intervertebral disc prosthesis is the CHARITÉ™ Artificial Disc which was the first disc approved for disc replacement use in the general population of back pain patients in the U.S. This prosthesis includes upper and lower prosthesis plates or shells which locate against and engage the adjacent vertebral bodies, and a low friction core between the plates. The core has upper and lower convexly curved surfaces and the plates have corresponding, concavely curved recesses which cooperate with the curved surfaces of the core. This allows the plates to slide over the core to allow required spinal movements to take place. The curved recesses in the plates are surrounded by annular ridges which locate, at the limit of sliding movement of the plates over the core, in opposing upwardly and downwardly facing, peripheral channels surrounding the curved surfaces of the core.

This type of disc configuration is described in U.S. Pat. No. 5,401,269. A drawback of such configurations is that because the core is in effect merely “clamped” between the plates, this configuration does not allow for secure retention of the core. In one alternative arrangement, the curved surfaces of the core carry opposing, elongate keys that locate in elongate grooves in the plates and another alternative arrangement in which the plates have opposing elongate keys that locate in elongate grooves in the opposite curved surfaces of the core. These key and groove arrangements allow the plates to slide over the core within the limits of the length of the grooves, in one direction only. Although allowance is made for some lateral play of the keys in the grooves, very little sliding movement of the plates over the core can take place in the orthogonal vertical plane, and this is considered to be a serious drawback of this design. Other currently available intervertebral disc prostheses have similar and/or other drawbacks. Typically, drawbacks include insufficient resistance to wear and tear, restricted range of motion, undesirable contact between plates causing potential wear, excessive disc height not appropriately matched to patient anatomy and/or insufficient ability of the prosthesis to adhere to vertebral bone.

Therefore, a need exists for improved intervertebral disc prostheses. Ideally, such improved prostheses would resist wear and tear, provide a desired range of motion, provide a low height and adhere well to vertebral bone. At least some of these objectives will be met by the present invention. At least some of these objectives will be met by the inventions described herein below.

2. Description of the Background Art

A variety of intervertebral disc designs are described in described in U.S. Pat. Nos. 7,531,001; 7,442,211; 7,753,956; 7,575,599; 8,764,833 and 9,011,544.

BRIEF SUMMARY OF THE INVENTION

The present invention provides alternative and improved apparatus and methods for performing intervertebral disc replacement procedures.

In accordance with one embodiment, an intervertebral prosthesis for insertion between adjacent vertebrae includes a first plate, a second plate and a core. The first plate has an outer surface locatable against a respective vertebrae, an inner curved surface, an annular perimeter surrounding the inner curved surface and a lateral portion between the annular perimeter surface and the outer surface. A first central projection extends from the inner curved surface of the first plate. The second plate has an outer surface locatable against a respective vertebrae, an inner curved surface, an annular perimeter surface surrounding the inner curved surface and a lateral portion between the annular perimeter and the outer surface. A second central projection extends from the inner curved surface of the second plate toward the first plate. The core has upper and lower curved surfaces complementary in shape to the inner, curved surfaces of the first and second plates to allow the plates to slide over the upper and lower surfaces of the core and a central opening for receiving the first and second central projections. The first and second central projections and central opening cooperate with one another to retain the core between the plates and limit motion of the first and second plate with respect to one another to prevent contact between the annular perimeter surfaces of the first and second plates during sliding movement of the plates over the core. The first and second central projections each have a diameter of about one half or less of a diameter of the central opening in the core.

In accordance with another embodiment, an intervertebral prosthesis for insertion between adjacent vertebrae includes a first plate, a second plate and a core. The first plate has an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface. The second plate has an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface toward the first plate. The core has upper and lower curved surfaces complementary in shape to the inner, bearing surfaces of the first and second plates to allow the plates to slide over the upper and lower surfaces of the core and a central opening for receiving the central projections of the first and second plates. The first and second central projections and central opening cooperate with one another to retain the core between the plates and to prevent contact between any portion of the first and second plates during sliding movement of the plates over the core. The first and second plates are capable of articulating with respect to one another about the core at least 5 degrees and no more than 8 degrees from a neutral position in each of the anterior/posterior and lateral directions.

In accordance with further embodiment, a method of retaining a core in an intervertebral prosthetic disc includes steps of providing a first plate having an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface; providing a second plate having an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface toward the first plate; and providing a core between the first and second plates. The core has upper and lower curved surfaces complementary in shape to the inner, bearing surfaces of the first and second plates to allow the plates to slide over the upper and lower surfaces of the core and a central opening for receiving the central projections of the first and second plates. The core is retained between the plates and contact is prevented between the plates during sliding movement of the plates over the core by cooperation of the first and second central projections and central opening when the intervertebral prosthetic disc is in an assembled configuration. The first and second plates are allowed to articulating about the core at least 5 degrees and no more than 8 degrees from a neutral position in each of the anterior/posterior and lateral directions in the assembled configuration.

According to another embodiment, a method of preventing contact between plates in an intervertebral prosthetic disc, the method comprising:

-   providing a first plate having an outer vertebral contacting     surface, an inner bearing surface and a central projection extending     from the inner bearing surface; -   providing a second plate having an outer vertebral contacting     surface, an inner bearing surface and a central projection extending     from the inner bearing surface toward the first plate; -   providing a core between the first and second plates, the core     having upper and lower curved surfaces complementary in shape to the     inner, bearing surfaces of the first and second plates to allow the     plates to slide over the upper and lower surfaces of the core and a     central opening for receiving the central projections of the first     and second plates; -   preventing contact between the plates during sliding movement of the     plates over the core by cooperation of the first and second central     projections and central opening when the intervertebral prosthetic     disc is in an assembled configuration; and -   allowing the first and second plates to articulating about the core     at least 5 degrees from a neutral position in each of the     anterior/posterior and lateral directions in the assembled     configuration, wherein the first and second central projections each     have a diameter of about one half or less of a diameter of the     central opening in the core.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which:

FIG. 1 is a cross-sectional view of a prosthetic disc in a neutral position;

FIG. 2 is a cross-sectional view of the prosthetic disc of FIG. 1 is a maximum articulated position;

FIG. 3 is a top view of the prosthetic disc of FIG. 1;

FIG. 4 is a side view of three different sizes of the upper endplate of the prosthetic disc of FIG. 1;

FIG. 5 is a bottom view of the three different sizes of the upper endplate of the prosthetic disc of FIG. 1;

FIG. 6 is a perspective view of an upper endplate of the prosthetic disc of FIG. 1;

FIG. 7 is a top view of an alternative embodiment of an upper endplate of a prosthetic disc;

FIG. 8 is a perspective view of another alternative embodiment of a prosthetic disc;

FIG. 9 is an anterior view of the prosthetic disc of FIG. 8;

FIG. 10 is a lateral view of the prosthetic disc of FIG. 8; and

FIG. 11 is a top view of the prosthetic disc of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 an intervertebral prosthesis 10 for insertion between adjacent vertebrae includes an upper plate 12, a lower plate 14 and a core 16. The core 16 is retained between the upper and lower plates 12, 14 by a retention feature in the form of central projections on the plates and an opening in the core 16. The retention feature is designed to allow the plates to slide over the upper and lower surfaces of the core in the anterior/posterior direction and in the lateral direction and to allow the plates to rotate with respect to each other and the core. The retention feature is also designed to prevent contact between the first and second plates during sliding movement of the plates over the core.

The upper plate 12 includes an outer surface 18 having a plurality of serrations 20 with serrations at outer portions of the plate having pyramid shapes and serrations at the inner portion of the plate having truncated pyramid shapes. Serrations between the outer and inner portions of the plate have been truncated to a lesser degree than the inner serrations. This variation in truncation allows the top of each of the serrations to lie in a single plane illustrated in FIG. 1 by the line X. The upper plate 12 has an inner bearing surface 22 with a concave spherical shape and a circular perimeter with a diameter D_(b) (shown in FIG. 5). A central projection 24 extends from a center of the inner bearing surface 20 toward the lower plate 14. The central projection 24 is in the form of a substantially cylindrical peg having a diameter D_(p) measured at substantially a midpoint of its height. The term substantially cylindrical as used herein means varying in diameter by less than 10 percent along at least 50 percent of a height or length. The upper plate 12 includes an annular perimeter surface 26 surrounding the inner curved bearing surface 22 on a bottom side of the plate. A lateral sidewall portion 27 extends around the plate 12 between the annular perimeter surface 26 and the outer vertebral body contacting surface 18.

The lower plate 14 has an outer surface 28 with a plurality of serrations 30 and an inner bearing surface 32 with a projection 34 extending upwards from the inner bearing surface. In the embodiment of FIG. 1, the lower plate 14 is identical to upper plate 12, however, in other embodiments the upper and lower plates can have different configurations to more closely match the anatomy of the patient. For example, the upper or superior plate may have a somewhat domed shaped outer surface to accommodate the anatomical concavity in the lower surfaces of the vertebrae. In addition, one or both of the upper and lower plates can be provided with non-parallel upper and lower surfaces to accommodate spinal lordosis. The advantages of identical upper and lower plates include the ease of manufacturing, assembly and insertion without need for checking orientation.

The core 16 is symmetrical with respect to a midline in each of three orthogonal dimensions. The central opening 36 of the core 16 has a smallest diameter D_(o) which is larger than the diameter D_(p) of the projections 24, 34. The diameter D_(p) of the projections is measured at approximately a middle of the projection about half way between a top and bottom of the projection. The central projections 24, 34 each have a diameter D_(p) of about 60% to 30% of a diameter D_(o), about 65% to 40% of a diameter D_(o), and preferably about one half or less of a minimum diameter D_(o) of the central opening in the core 16. The core has an overall diameter D_(c) which is substantially equal to the diameter D_(b) of the concave bearing surfaces of the upper and lower plates. In one embodiment, the diameter D_(c) of the core is about 95% to about 105% of the diameter D_(b) of the bearing surface. Although the core 16 is illustrated with a central opening 36 extending through the height of the core, the central opening can also be a partial opening not extending fully through the core. The diameter D_(o) of the central opening 36 of the core 16 is at least 30%, or preferably at least one third of a maximum diameter D_(c) of the core.

The core 16 has a shape of an annular, flattened ring with a cross section through the wall of the ring having a substantially isosceles trapezoidal shape with rounded corners. The core cross section has a long base side at the edge of the central opening 36 and a short base side at the exterior cylindrical wall of the core. Upper and lower surfaces of the core form the angled sides of the trapezoidal cross section and are preferably identical. The core shape can also be described as a flattened torus shape. Inner and outer walls of the core are substantially cylindrical and upper and lower walls of the core are portions of a sphere. Alternatively, the core 16 can be described as a perforated lens. The top and bottom surfaces are spherical with spherical radii's matching those of the congruent endplate concavities 22, 32.

The core has a maximum height H_(c) in the axial direction and a maximum diameter D_(c) in the radial direction. In one embodiment, the maximum diameter D_(c) is at least two times the maximum height H_(c).

FIG. 2 shows the intervertebral prosthetic disc at a maximum angle of articulation. At the maximum articulation, the projections 24, 34 abut the side walls of the opening 36 in the core and prevent further articulation. The core configuration as described and shown allows the upper and lower plates to articulate about the core at least 5 degrees and preferably about 6 degrees from a neutral position, shown in FIG. 1 in each of the anterior/posterior and lateral directions. The maximum articulation angle between the plates 12, 14 is the angle A, as shown in FIG. 2. The articulation angles A of 5 degrees and 6 degrees correspond to a total anterior/posterior or lateral articulation of about 10-12 degrees between endplates 12, 14. Preferably a maximum articulation angle A between the plates 12, 14 is less than 8 degrees in any direction for a total articulation of 16 degrees or less.

The central projections 24, 26 and central opening 36 cooperate with one another to retain the core between the plates and limit motion of the first and second plate with respect to one another to prevent contact between the annular perimeter surfaces 26 of the plates during sliding movement of the plates over the core. As shown in FIG. 2, a gap B between the annular perimeter surfaces 26 at a maximum articulation is preferably at least 0.5 mm. The prevention of contact between the plates 12, 14 significantly reduces metallic wear particulate generation in wear testing and in vivo which can be caused by rubbing contact between metallic plates. Preferably, the central projections 24, 34 and central opening 36 create both a retention feature to retain the core between the plates and a motion limiting feature to prevent contact between the annular perimeter surfaces 26 and between any other part of the plates 12, 14 during sliding movement of the plates over the core.

The assembled disc in the neutral configuration shown in FIG. 1 with the plates in a parallel orientation and the core positioned centrally has a height H_(d) which can vary between 3.5 mm and 8 mm for a cervical disc and 8 and 15 mm for a lumbar disc. In a preferred embodiment of a cervical disc, the disc is provided in a range of heights including at least 3 different heights from 3.5 to 6 mm. For example, heights 4, 5 and 6 mm can be provided to accommodate different patient anatomies. The different height discs may each include the same core size and different plate heights. Alternatively, the same plate heights can be used for all sizes with different core heights, or both the heights of cores and plates can vary between sizes. In a preferred set of discs, at least one disc size has a maximum height between outer surfaces of the two plates of 5 mm or less to accurately match anatomical heights.

In a preferred embodiment, the total maximum disc height H_(d) is less than 2 times a height H_(c) of the core and more preferably, the core maximum height H_(c) is 55% or more of the total disc height H_(d).

Interchangeable cores can also be provided which provide the surgeon with options for providing more or less motion depending on the motion desired for a particular patient. In one example, a first core having a central opening 36 having a diameter D_(o) of the core 16 of about two times a diameter of the central projection 24 provides standard 10-18 degree motion, while a second core having a central opening 36 having a diameter D_(o) of about 1.5-1.8 times a diameter or the central projection 24 provides limited motion of 5-12 degrees. The cores may be interchangeable prior to implantation. Alternately, the cores may be interchanged after the initial surgery with a follow on surgery to increase or decrease motion without removing the upper and lower plates. In one embodiment, a set of parts is provided as a surgical set for assembling an intervertebral disc for implantation in a patient, the set of parts can include identical or different upper and lower endplates and a plurality of cores having the same spherical curvature of upper and lower core bearing surfaces and different central opening diameters D_(o).

In one embodiment, the upper and lower plates are formed of a metallic material, such as but not limited to, cobalt chrome molybdenum, titanium, composites of metal and ceramic and/or the like. The bone contacting surfaces of the upper and lower plates can be roughened or treated such as by aluminum oxide blasting or coated, such as with pure titanium, HA (hydroxylapatite) coating, micro HA coating, and/or bone integration promoting coatings. Any other suitable metals or combinations of metals may be used as well as ceramic or polymer materials, and combinations thereof. The bearing surfaces can be uncoated or treated or coated, such as, coated with titanium nitride. In some embodiments, it may be useful to couple two materials together to form the inner surface and the outer surface of the plates. Any other suitable combination of materials and coatings may be employed in various embodiments of the invention.

The core can be formed of a low-friction material, such as biologically compatible polymers including polyethylene, PEEK, UHMWPE, Vitamin E stabilized UHMWPE, PLA, fiber reinforced polymers, ceramics, metals, composites or the like. In one example, the core can be formed of PAEK materials including neat (unfilled) PEEK, PEEK-OPTIMA available from Invibio, Inc., fiber reinforced PEEK, such as PEEK-CFR (carbon fiber reinforced) from Invibio, Inc., glass fiber reinforced PEEK, ceramic filled PEEK, Teflon filled PEEK, barium sulfate filled PEEK or other reinforced or filled PAEK materials.

According to one embodiment of the invention, the upper and lower plates are formed of titanium and the core is formed of polyethylene to provide a low wear metal on poly bearing without metal on metal contacting surface.

FIGS. 3-6 show a single upper or lower plate or a series of plates of differing sizes. As seen most clearly in the top view of FIG. 3 and the bottom view of FIG. 5, each of the plates has a pair of side notches 100 configured for grasping the plate with an insertion instrument. As shown in FIGS. 3-6, the side notches 100 are located toward the anterior side of the plates for grasping the plates on the anterior side of the disc and insertion from the anterior side of the spine. The insertion instrument (not shown) includes opposing jaws which fit into the two side notches 100. In one preferred embodiment, a single pair of opposition jaws is able to grasp both the upper and lower plates by engaging the upper and lower plates both with the same opposing right and left jaws. Once the upper and lower plates are assembled with the core there between and grasped by the insertion instrument, the upper and lower plates and core are held in a fixed configuration and not able to articulate until released from the jaws of the insertion instrument. In the constrained insertion configuration, the upper and lower plates can be parallel or slightly angled with the posterior edges of the plates closer to one another.

As shown in FIGS. 4 and 5, the discs may be provided in different sizes, such as large 12 a, medium 12 b and small 12 c footprint sizes having different width and depth dimensions to fit different patients. For the purpose of using the same instrument to insert all of the different sizes, the shapes and sizes of the anterior side of each of the discs and the notches 100 are identical or at least have the same notches.

The serrations 20 have a pyramid shape. As shown in the top view of FIG. 3, the pyramid shaped serrations can be elongated in the lateral direction to provide improved plate securement in the anterior posterior direction. The anterior/posterior direction is the direction most likely to have plate migration. In one example, the ratio of anterior/posterior dimension to lateral dimension of the serrations is between about 2:3 to about 3:4.

In the embodiments shown, the pyramid shaped serrations have been included for improving fixation. However, other types of fixation may also be included in addition to or in place of the serrations, such as teeth or fins. For example, a single central fin can be provided on each of the plates extending in an anterior posterior direction with an angled posterior edge for aiding in insertion. Alternatively, two or more fins can also be provided on each plate. The fins can be configured to be placed in slots in the vertebral bodies or to be placed without cutting slots. In one example, a single fin can be provided on one plate while double fins can be symmetrically arranged on the other plate to achieve a staggered arrangement particularly useful for multi-level disc implant procedures. Multiple small fins can also be provided in an in-line orientation. In alternative embodiments, the fins may be rotated away from the anterior-posterior axis, such as in a lateral-lateral orientation, a posterolateral-anterolateral orientation, or the like for implantation in the associated directions.

FIG. 7 shows an alternative embodiment of an upper endplate 112 having a different configuration of projections on a bone contacting upper surface 118. A plurality of different shaped serrations are provided including symmetrical serrations 120 about the periphery of the disc, laterally elongated serrations 122 on a central dome 124, non-symmetrical serrations 126 around the periphery of the central dome, and serrations (or teeth) 128 with a vertical or near vertical anterior surface for additional fixation at the anterior side of the plate. The endplate 112 also has an anterior pair of side notches 100 configured for grasping the plate with an insertion instrument. Any one or more of these serrations types can be combined with the elongated pyramid shaped serrations 20 of the FIG. 1-6 embodiment.

FIGS. 8-11 show a further alternative embodiment of a pair of matching upper and lower plates 212, 214 positioned about a core 216. In this embodiment, a pair of side notches 200 configured for grasping the plate with an insertion instrument are positioned at a centered location between the anterior and posterior edges of the endplates. In this embodiment with centrally located notches 200, different size discs are inserted with different insertion instruments or the insertion instrument is adjustable to accommodate the different plate sizes. This embodiment has upper and lower plates 212, 214 which are identical and symmetrical and thus can be assembled without respect to top/bottom or anterior/posterior orientations.

Modification of the above-described assemblies and methods for carrying out the invention, combinations between different variations as practicable, and variations of aspects of the invention that are obvious to those of skill in the art are intended to be within the scope of the invention disclosure. 

What is claimed is:
 1. An intervertebral prosthesis for insertion between adjacent vertebrae, the prosthesis comprising: a first plate having an outer surface locatable against a respective vertebrae, an inner curved surface, an annular perimeter surface surrounding the inner curved surface and a lateral portion between the annular perimeter surface and the outer surface, wherein a first central projection extends from the inner curved surface; a second plate having an outer surface locatable against a respective vertebrae, an inner curved surface, an annular perimeter surface surrounding the inner curved surface and a lateral portion between the annular perimeter and the outer surface, wherein a second central projection extends from the inner curved surface toward the first plate; a flattened ring shaped core having upper and lower surfaces complementary in shape to the inner, curved surfaces of the first and second plates to allow the plates to slide over the upper and lower surfaces of the core and a central opening for receiving the first and second central projections, wherein the core has a cross section having an isosceles trapezoidal shape; wherein the first and second central projections and central opening cooperate with one another to retain the core between the plates and limit motion of the first and second plate with respect to one another to prevent contact between the annular perimeter surfaces of the first and second plates during sliding movement of the plates over the core; and wherein the first and second central projections each have a diameter of about one half or less of a diameter of the central opening in the core.
 2. An intervertebral prosthesis of claim 1, wherein the core has a maximum height in the axial direction and a maximum diameter in the radial direction and wherein the maximum diameter is at least two times the maximum height.
 3. An intervertebral prosthesis of claim 1, wherein the diameter of the central opening of the core is at least one third of a maximum diameter in a radial direction of the core.
 4. An intervertebral prosthesis of claim 1, wherein a maximum height of the intervertebral prosthesis in a neutral position with the first and second plates parallel and the core positioned between the plates is 5 mm or less.
 5. An intervertebral prosthesis of claim 1, wherein the first and second plates are capable of articulating about the core at least 5 degrees from a neutral position in each of the anterior/posterior and lateral directions.
 6. An intervertebral prosthesis of claim 1, wherein the first and second central projections and central opening cooperate with one another to retain the core between the plates and limit motion of the first and second plate with respect to one another to prevent contact between any part of the first and second plates during sliding movement of the plates over the core.
 7. An intervertebral prosthesis of claim 1, wherein the core cross section has a long base within the central opening and a short base on the exterior of the core.
 8. An intervertebral prosthesis of claim 7, wherein the core cross section has first and second curved corners between the long base within the central opening and the upper and lower surfaces and third and fourth curved corners between the short base on the exterior of the core and the upper and lower surfaces.
 9. An intervertebral prosthesis of claim 8, wherein the core cross section has only the first, second, third and fourth corners and no additional corners.
 10. An intervertebral prosthesis of claim 1, wherein the central opening in the core is a through opening.
 11. An intervertebral prosthesis of claim 1, wherein the outer surfaces of the first and second plates have pyramid shaped serrations which are elongated with a larger dimension extending in a lateral direction and a smaller dimension extending in an anterior/posterior direction.
 12. An intervertebral prosthesis of claim 11, wherein the pyramid shaped serrations are rectangular in shape.
 13. An intervertebral prosthesis for insertion between adjacent vertebrae, the prosthesis comprising: a first plate having an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface; a second plate having an outer vertebral contacting surface, an inner bearing surface and a central projection extending from the inner bearing surface toward the first plate; a flattened ring shaped core having upper and lower surfaces complementary in shape to the inner, bearing surfaces of the first and second plates to allow the plates to slide over the upper and lower surfaces of the core and a central opening for receiving the central projections of the first and second plates, wherein the core has a cross section having an isosceles trapezoidal shape; wherein the first and second central projections and central opening cooperate with one another to retain the core between the plates and to prevent contact between any portion of the first and second plates during sliding movement of the plates over the core; and wherein the first and second plates are capable of articulating with respect to one another about the core at least 5 degrees and no more than 8 degrees from a neutral position in each of the anterior/posterior and lateral directions.
 14. An intervertebral prosthesis of claim 13, wherein the central projections of the first and second plates are substantially cylindrical.
 15. An intervertebral prosthesis of claim 13, wherein the central projections of the first and second plates each have a diameter of about one half or less of a diameter of the central opening in the core.
 16. An intervertebral prosthesis of claim 13, wherein the core has a maximum height in the axial direction and a maximum diameter in the radial direction and wherein the maximum diameter is at least two times the maximum height.
 17. An intervertebral prosthesis of claim 13, wherein a diameter of the central opening of the core is at least one third of a maximum diameter of the core in a radial direction.
 18. An intervertebral prosthesis of claim 13, wherein a maximum height of the intervertebral prosthesis in a neutral position with the first and second plates parallel and the core positioned between the plates is 5 mm or less.
 19. An intervertebral prosthesis of claim 13, wherein the central opening in the core is a through opening.
 20. An intervertebral prosthesis of claim 13, wherein the outer vertebral contacting surfaces of the first and second plates have pyramid shaped serrations which are elongated with a larger dimension extending in a lateral direction and a smaller dimension extending in an anterior/posterior direction.
 21. An intervertebral prosthesis of claim 20, wherein the pyramid shaped serrations are rectangular in shape.
 22. An intervertebral prosthesis of claim 13, wherein the core cross section has first and second curved corners between a long base within the central opening and the upper and lower surfaces and third and fourth curved corners between a short base on the exterior of the core and the upper and lower surfaces.
 23. An intervertebral prosthesis of claim 22, wherein the core cross section has only the first, second, third and fourth corners and no additional corners. 